Flexible pipes of the present type for offshore transportation of fluids are well known in the art and are for example described in “Recommended Practice for Flexible Pipe”, API 17 B, second Edition, 1998, which provides a standard for such pipes. Such pipes usually comprise an innermost sealing sheath also often called an inner sealing sheath, an inner sheath or an inner liner, which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armor layers on the outer side of the inner sealing sheath (outer armor layer(s)). An outer sealing sheath may be provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armor layers. The innermost sealing sheath defines a bore of the unbonded flexible pipe in which a fluid can be transported.
Typical unbonded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. Nos. 6,123,114, 6,085,799 and in Recommended Practice for Flexible Pipe, API, 17B, Fourth edition, July 2008, and in Specification for Unbonded Flexible Pipe, API, 17J, third edition, July 2008.
The term “unbonded” means in this text that at least two of the layers including the armor layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armor layers located outside the inner sealing sheath. These armor layers are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
In order to have sufficient strength, in particular to prevent the collapse of the inner sealing sheath, the flexible pipe often comprises an armor layer located inside the space defined by the inner sealing sheath. Such inner armoring layer or layers are normally referred to as a carcass and are mainly provided in order to reduce the risk of collapse of the pipe when used under high hydrostatic pressure. An unbonded flexible pipe with a carcass is usually referred to as a rough bore pipe. The prior art carcasses are usually made from helically wound and interlocked metal strips, such as for example described in U.S. Pat. Nos. 6,904,939 and 6,145,546. However, also the use of profiled elongate elements in a carcass has been suggested in EP 1 475 560 and WO 2008/077409 by the assignee.
The innermost sealing sheath is normally extruded directly onto the carcass optionally with a thin tape e.g. fibrous tape between the layers. Since the material of the innermost sealing sheath at the extrusion stage is relatively soft and plastic it has a tendency to creep into helically shaped gaps provided by the helically wound and interlocked metal strips of the carcass. This creep into the helically shaped gaps can be very damaging to the innermost sealing sheath and often results in notch formation in the innermost sealing sheath which provides weakness to the innermost sealing sheath. In certain situations, in particular when the internal or external pressure is relatively high, such creep may result in a growing notch formation which results in reduced durability and even burst of the pipe. In order to avoid said creep prior art solutions have been provided, where a sacrificial layer or a rod has been applied between the carcass and the innermost sealing sheath in order to protect the innermost sealing sheath against damage. Such solutions are for example described in U.S. Pat. Nos. 6,843,278 and 6,145,546.
Another way of solving the problem of creep of the innermost sealing sheath is to provide the carcass with very reduced helically gap or gaps e.g. using shaped profiles instead of exclusively folded strips e.g. as disclosed in EP 1 475 560, DK PA2011 00037 and DK PA 2011 00099. By providing the carcass from profiles e.g. held together using strips, the surface of the carcass facing the innermost sealing sheath can be formed as a supporting surface with very small gaps between windings such that the innermost sealing sheath practically does not creep into such gaps. This solution has, however, found to have another drawback where the flexible pipe is intended for use as a riser. It has been found that in worst case the carcass can slip under its own weight resulting in increased gaps in the uppermost part of the flexible part and loss of flexibility in the lower part of the flexible pipe and ultimately such slip may result in total damage of the flexible.